Steady froth formation is an undesirable phenomenon occurring in aerating media containing organic matter. Stability of froth is associated with the composition of complex media or products of metabolism of microorganisms which are not identified in many instances, and the fight against froth formation is frequently conducted on an empirical basis.
Negative effect of froth formation consists in the following:
a culture with froth may be let out of the fermenter through air outlet ports; PA1 the value of KL.sub.a (efficiency of oxygen absorption) decreases; PA1 in chemostatic cultures (cultures with complete stirring to which a culture medium is added at a constant rate and from which a culture is taken-off at the same rate while retaining the total volume unchanged) the gas content changes, hence the liquid volume becomes uncontrollable. PA1 elimination of froth-forming substances and action on the froth with physico-chemical facilities; PA1 destruction of froth by mechanical, hydro- and aerodynamic methods; PA1 acoustic froth suppression using audio and ultrasonic frequency oscillations; PA1 thermal froth suppression using saturated steam or heated liquid; PA1 electrical froth suppression; PA1 stabilization of froth level and temporary suspension of air supply for aeration or temporary suspension of mechanical stirring or discharge of excessive froth from the apparatus; PA1 combined methods.
Froth formation is generally prevented by using froth suppressors which may be classified in the following manner:
Among all the above-mentioned froth suppressors only chemical and mechanical ones have found a widespread use in the microbiological industry since all remaining froth suppressors are poorely studied and not applied in practice for various reasons. Thus, the works on the use of ultrasonic oscillations for froth suppression are still at the laboratory stage and, according to preliminary estimations, this method for suppressing large quantities of froth in fermenters are bound to prove economically inefficient. The use of thermal froth suppressors is limited by the sensitivity of many microorganisms to high temperature, while the knowledge of the influence of electric field, especially of .alpha.-particles on microorganisms is still inadequate so that their use in the microbiological applications is limited.
Various mechanical froth suppressors are recommended.
One of the simplest modifications consists in using a rapidly rotating rotor (cf. British patent specification No. 8,922,505, Cl. 14(2)L, publ. in 1962).
Such froth suppressors are generally mounted in a confining device of the cyclone type which can be connected to a vacuum chamber.
The prior art teaches a mechanical froth suppressor comprising a perforated plate having two impellers installed thereon of which one--the upper impeller--is arranged with the vanes up and the other--the lower impeller--is arranged with the vanes down. For destructing froth over the liquid surface, there is provided a turbine rotating about its axis so that during rotation of the turbine an ascending gas flow is formed under the turbine, which takes-in the froth to be thrown away by the turbine blades in the form of liquid jets. The working member comprises a perforated plate submerged in the froth and connected with a vibratory drive (cf. U.S. Pat. No. 2,610,155, publ. in 1962). Known in the art is an apparatus for mechanical froth suppression, comprising a rotary perforated plate having partitions in the form of hollow truncated cones mounted on the plate with spaces therebetween for the passage of liquid (cf. USSR Inventor's Certificate No. 107900, Cl. C 12B 1/18, publ. in the Off. Bull. No. 9, 1957, page 16).
An apparatus for froth suppression, comprising a stack of conical plates on a hollow shaft is widely used in practice (cf. Swiss Pat. No. 1660, publ. in 1968).
The apparatus is installed on an independent shaft. The apparatus are used both in laboratory and commercial installations.
Known in the art is a mechanical blade froth suppressor with a horizontally extending working shaft. To improve the efficiency of froth suppression, a partition wall is provided inside the froth suppressor vessel to divide the vessel into two communicating compartments, one compartment having at the bottom thereof a pipe having its inlet end arranged immediately under impellers, and the outlet end is incorporated in the partition wall (cf. USSR Inventor's Certificate No. 246446, Cl. C 12C, publ. in the Off. Bull. No. 21, 1969, page 10).
A hydraulic and pneumatic system for collecting, removing and destructing froth was contemplated, featuring recirculation of concentrated solution and having two pumps. Froth suppression is effected by means of pumps, one pump being designed for processing gas and liquid emulsions. A portion of waste gas is fed for recirculation for pneumatic froth suppression. The second pump takes-off the liquid from the settling basin and also feeds it for recirculation (cf. U.S. Pat. No. 3,339,345, Cl. 55-178, publ. in 1967).
All above-described froth suppressors are rarely used in the microbiological applications in spite of their large structural variety. Such apparatus either cannot provide for complete suppression of froth or they impose considerable power requirements. In addition, such systems are cumbersome and do not permit the useful space of the fermenter to be completely utilized so that the filling ratio of fermenters is from 0.5 to 0.6 of the total volume. The free space is used for compensation of the level rise which generally does not exceed 10% owing to gas content after the aeration is put on, and also for controlling the froth level.
Automatic control of processes occurring during cultivation of microorganisms, including the froth suppression process, is very important for operation of fermenters. In principle, this problem should be very simple to resolve: a mechanical stirrer and an actuator for adding a chemical froth suppressor are to be mounted in the fermenter, the devices being operable by means of a mechanism actuated in response to the presence of froth above an admissible level. A simple solution resides in an installation of a sensor--a contact electrode--at a preset level producing a signal when the froth is in contact with the electrode. Not every froth is, however, electrically conducting. A-c or d-c voltage of 10-30 V is fed for the electrode supply and a current of 5 to 20 mA and over flows through the froth. In case d-c supply is used, such voltage and amperage cause polarization of the electrode so that its sensitivity appreciably changes during fermentation. The use of alternating current results in erosion of the electrode, while metal ions getting to the culture liquor negatively affect the process of cultivation of microorganisms.
The use of electrodes is not always justified as a comparably large area of contact of the forth with electrode is required, and the froth sticking to the electrode causes changes in its operating parameters.
Therefore, various contactless froth level indicators have been contemplated. Photocells are in a widespread used (cf. USSR Inventor's Certificate No. 128827, Cl. C 12 B 1/18, publ, in the Off. Bull., No. 11, 1960) incorporated in the fermenter wall, which, after the froth cuts-off the light beam, turn on actuators, such as an electric motor of a gear pump for feeding a chemical suppressor.
Culture media generally contain various quantities of mineral salts which, after dissolution, form relatively well electrically conducting froth having various electrical resistance. Therefore a contact electrode was contemplated for indicating the froth level (cf. M. Zh. Kristapsons, L.Ya. Latsis, Coll, or Art. "Controlled Microbeal Synthesis" (in Russian), Riga, Znanie Publ., 1973), to be inserted in an arm of a bridge circuit. This enables the electrode supply with an a-c voltage of 0.2 V so that maximum current flowing through the froth does not exceed 100 .mu.A. Such amperage and voltage cannot have any negative influence in the microbiological process and life activity of microorganisms.
An interesting solution involves the installation of a contact electrode on a float which is movable up and down depending on the froth level (cf. GDR Pat. No. 76454, Cl C 12 B, publ. in 1970).
The use of apparatus for chemical froth suppression enables more efficient control of froth formation thereby improving the filling ratio of a fermenter. It should be, however, noted that the control of froth suppressor flow rate requires additional systems which are sophisticated expensive in the manufacture and do not always comply with the requirements imposed by microorganism cultivation conditions; besides they do not exclude overconsumption of froth suppressor.
Also known in the art is an apparatus for chemical froth suppression in a fermenter having a froth sensor tracing the froth in the fermenter, and a vessel containing a chemical suppressor which is fed to the fermenter by means of a pneumatic pump along a take-off pipe (cf. Technical Description and Operation Manual "Complex of Cultivation Equipment" (in Russian), SKB Biologicheskogo priborostroenia AN SSSR, Pushchino, 1978, pp. 37-40).
In this apparatus, the froth sensor comprises a capillary tube installed in the interior of the fermenter at a preset level and connected by means of a pipeline to a sensor member having a magnetic contact. Another pipeline connecting the sensor member to the interior of the fermenter is provided with a pump for pumping air along the resultant closed circuit.
The apparatus also comprises an actuating mechanism having a vessel containing a chemical froth suppressor provided with a take-off pipe having a pinch valve for feeding the froth suppressor from the vessel containing the chemical froth suppressor to the fermenter following a signal from the sensor member.
The apparatus functions in the following manner.
When froth in the fermenter does not touch the opening of the capillary tube, the pump pumps a gas mixture from the fermenter through the capillary tube and sensor member to return it back to the fermenter. The sensor member is so adjusted that its membrane having a permanent magnet secured thereto is stationary, the magnetic contact is open, and the pinch valve of the actuating mechanism is closed. When the froth approaches the opening of the capillary tube, the pump starts feeding the froth, the resistance of the capillary tube abruptly increases so that the pump causes a pressure reduction in the sensor member. The membrane starts displacing until the magnetic contact is closed, to open the pinch valve for feeding a froth suppressor. The froth suppressor is admitted to the fermenter to destruct the froth thereby cleaning the capillary tube so that the resistance of the capillary tube again decreases, and the sensor member returns back to its initial position to open the magnetic contact. The pinch valve is again closed to interrupt the admission of the chemical froth suppressor to the fermenter.
This apparatus for chemical froth suppression may be used in both commerical and laboratory fermenters.
The provision of the gas circulation circuit passing through the sensor member which becomes a focus of decay for microorganisms when clogged with froth breaks the septic conditions of microbiological process. In addition, pulse feeding of froth suppressor to the fermenter does not exclude overconsumption of froth suppressor as the process of froth formation cannot be forecast by a researcher. The absence of stirring of froth suppressor causes its stratification thereby changing the character of its action on the froth layer.